This invention is related to co-pending U.S. application entitled "KNOCK DETECTING APPARATUS FOR INTERNAL COMBUSTION ENGINES" Ser. No. 178,282 filed on Aug. 15, 1980 in the names of the same inventors and assigned to the same assignee of this application.
The present invention relates to a knock detecting apparatus for use with engine ignition timing control apparatus, etc., which is designed to function such that the presence of knock is detected from the vibrations produced outside the engine cylinders due to the cylinder pressure and the ignition timing is controlled to obtain the desired knock intensity.
It is well known in the art that there is a close correlation between the ignition timing and the cylinder pressure.
When a mixture is exploded, in the absence of knock there occurs no superposition on the cylinder pressure of any higher harmonic components (usually those frequency components which fall in the range of 4 to 10 kHz and/or the range of 11 to 13 kHz and which are mainly determined by the engine cylinder bore diameter and the intermittent rapid combustion rate), whereas when knocking occurs the superposition of such higher harmonic components on the cylinder pressure begins to take place at around the maximum cylinder pressure and this results in the generation of vibrations or sounds outside the cylinders. A careful examination of the generating conditions of the pressure signals produced inside the cylinders or the vibrations or sounds produced outside the cylinders shows that the beginning of knock (or trace knock) starts to occur at an engine crank angle at which the cylinder pressure attains the maximum value. Then, as the knock increases in intensity gradually (to light knock and heavy knock), the higher harmonic components start to superpose on the cylinder pressure considerably earlier (or on the ignition side) than the maximum cylinder pressure crank angle. Thus, if the vibrations or sounds produced outside the cylinders by knocking are detected accurately and fed back to control the ignition timing, the engine efficiency will be improved considerably. However, presently no detecting apparatus have been developed which are capable of accurately detecting the presence of knock or a feedback factor and also capable of stably operating under such severe environmental conditions as required for vehicles.
Known knock detecting apparatus of the above kind include two types one of which employs a piezoelectric accelerometer which is usually used for vibration detecting purposes and whose frequency characteristic is higher than the knock frequencies and flat below the resonant point or frequency (this type is hereinafter referred to as a non-resonant type detecting apparatus). The other type employs a resonant detecting apparatus of the kind which has been investigated by the inventors, etc., and whose resonant characteristic is adjusted to the knock frequencies. Since the resonant point of the non-resonant type detecting apparatus is higher than the knock frequencies, the sensitivity of the apparatus is substantially constant in the lower frequency range including the knock frequencies lower than the resonant point. Thus, as a matter of principle, the apparatus is capable of detecting the presence of knock over the entire knock frequency range. However, when the engine is in operation, vibration noise such as one caused by the seating of the valves occurs frequently and the S/N ratio between the vibration noise and the knock is deteriorated, thus making it practically impossible for the non-resonant type detecting apparatus to detect the presence of knock at high engine speeds. Moreover, the detection sensitivity is low and the detection of weak knock is difficult even at low engine speeds.
In the case of the resonant type detecting apparatus, the detection sensitivity is improved considerably with respect to the particular frequencies around the resonant point and the superposition of vibration noise of other frequencies becomes difficult, thus greatly improving the S/N ratio and sensitivity with respect to knocking.
However, the resonant type detecting apparatus has many problems from the strength point of view due to the strong vibrations of the vibrating member at the resonant frequency and due to the application of the strong engine vibrations to the vibrating member, and the apparatus also has the disadvantage of the vibrating member being difficult to manufacture.